The present invention relates to a metallic additive free electroplating bath for the deposition of white palladium metal on various surfaces and, in particular, to the use of an unsaturated sulfonic compound in combination with a special class of pyridine related nitrogen compounds in the bath to both stabilize the bath and to provide whiter palladium deposits over a wider range of plating thicknesses than conventional processes.
White colored finishes on decorative items such as jewelry are usually of silver, rhodium, palladium or their respective alloys. Each of these plated surfaces has its own specific drawbacks however, since silver surfaces lack durability due to tarnishing, rhodium processes are inefficient and costly to use and conventional palladium deposits do not have the pleasing white appearances of either rhodium or silver.
Several earlier patents on plating palladium address these problems and use small amounts of organic and/or metallic brighteners in the palladium bath to impart the desired color and brightness to the palladium. Deposits obtained from such baths still lack the specularity and whiteness of silver or rhodium however, and as the plated thickness exceeds about 1 micron the palladium gradually loses its brilliance. Moreover, metallic brighteners (i.e., Co or Ni) may cause an allergenic reaction with the skin and the use of these additives is objectionable to the jewelry industry.
Electroplating baths designed to improve the brightness of palladium or palladium alloy deposits on metal substrates are known in the art. See, for example, U.S. Pat. No. 4,098,656 which issued to Deuber in 1978. In this patent the improved brightness is achieved by utilizing in the bath both a Class I and a Class II organic nickel brightener and an adjusted pH range from 4.5 to 12. U.S. Pat. No. 4,406,755 to Morrissey relates to bright palladium electroplating solutions and utilizes aqueous solutions containing palladium complexed with an organic polyamine, and also containing both a cyclic organic imide and a nitrogen-containing heterocyclic organic compound at least one nitrogen of which is incorporated into a six-membered ring.
U.S. Pat. No. 4,487,665 to Miscioscio et al. discloses that thin, white palladium metal deposits can be readily obtained from very specific electroplating bath formulations containing a bath soluble source of palladium and a bath soluble ammonium conductivity salt, chloride ions, and a brightener from the groups of organic and inorganic brighteners, preferably the combined use of both an organic and an inorganic brightener such as 2-Formylbenzene sulfonate, sodium salt and nickel sulfate.
The need still exists for electroplating baths which provide a white palladium metal deposit without the need for metallic brighteners such as nickel and cobalt and it is an object of the present invention to provide a stable palladium electroplating solution wherein the deposits are bright in appearance at thicknesses above 1 micron and preferably up to about 5 microns.
Further objects and advantages will be apparent from the following description.